Respiratory syncytial virus (RSV) is a significant public health concern. It is the major cause of respiratory disease in infants. It also causes severe disease in individuals who are immunocompromised, elderly or have underlying lung disorders, such as asthma or chronic obstructive pulmonary disease. Currently there is no vaccine or effective antiviral drug to treat patients infected with RSV. Treatment is limited to supportive care, which in severe cases requires hospitalization and intensive care. Thus, there is a clear need for anti-RSV drugs that limit the severity of RSV disease to reduce the mortality associated with RSV, as well as the need for intensive treatment. An ideal target for antiviral drugs is the RSV RNA dependent RNA polymerase (RdRp), as it has unique and essential enzymatic activities. The RdRp transcribes and replicates the non-segmented negative sense RNA genome and is capable of initiating both activities from the same viral promoter. The goal of this project is to generate detailed molecular information about key functional and structural properties of the RSV RdRp. The work is divided into three major aims. The experiments in Aim 1 will elucidate the mechanisms by which the RdRp initiates transcription and RNA replication from two sites within the same promoter element, and determine the likelihood of using small molecules to successfully inhibit these steps. Aim 2 will determine how the RdRp is assembled into fully-competent transcriptase and replicase complexes, and will investigate how these complexes are regulated. The research in Aim 3 will utilize negative stain and cryo-electron microscopy to gain insight into the structural characteristics of the RdRp alone and in association with promoter RNA and the nucleocapsid. This research will significantly improve our understanding of the functional and structural properties of the RdRp and provide insights that could be exploited for rational development of antiviral drugs.